Blood: Structure, Function, and Physiology

Introduction to Blood

Blood is a specialized bodily fluid that plays a vital role in the circulatory system, transporting substances, defending against disease, and maintaining homeostasis. It is the only liquid connective tissue in the body, composed of formed elements suspended in plasma.

• Volume: Approximately 5-6 liters in males, 4-5 liters in females (about 8% of total body mass).

• Components: Formed elements (cells and cell fragments) and plasma (non-living fluid matrix).

• Functions: Transport of oxygen, nutrients, hormones, and wastes; protection against blood loss and infection; regulation of temperature, pH, and fluid balance.

Major Components of Blood

Blood consists of plasma and formed elements. The formed elements include erythrocytes, leukocytes, and platelets.

• Erythrocytes (Red Blood Cells, RBCs): Oxygen-transporting cells; most abundant formed element.

• Leukocytes (White Blood Cells, WBCs): Immune function and defense; five types divided into granulocytes and agranulocytes.

• Platelets: Cell fragments involved in blood clotting.

Formed Elements Classification

• Granulocytes: Neutrophils, Eosinophils, Basophils (contain visible granules).

• Agranulocytes: Lymphocytes, Monocytes (lack visible granules).

Blood Plasma

Plasma is the liquid portion of blood, making up about 55% of total blood volume. It is a complex mixture of water, proteins, nutrients, electrolytes, nitrogenous wastes, hormones, and gases.

• Water: ~92% of plasma.

• Plasma Proteins: Albumins, Globulins, Fibrinogen.

• Serum: Plasma without fibrinogen (after clotting).

Major Plasma Proteins

• Albumins: Smallest and most abundant; contribute to viscosity, osmolarity, and influence blood pressure and fluid balance.

• Globulins: (Alpha, beta, gamma) Transport proteins and antibodies; provide immune system functions.

• Fibrinogen: Precursor to fibrin, essential for blood clotting.

Physical Properties of Blood

• Hematocrit: Percentage of blood volume that is RBCs (about 45%).

• Buffy Coat: Thin layer containing WBCs and platelets (~1%).

• Plasma: Remainder of volume (~55%).

Functions of Blood

• Transport: Oxygen, carbon dioxide, nutrients, hormones, and waste products.

• Protection: Clotting (hemostasis) and immune defense.

• Regulation: Body temperature, pH (buffering via bicarbonate), and fluid balance.

Equation for Bicarbonate Buffering:

Erythrocytes and Oxygen Transport

Structure and Function of Erythrocytes

Erythrocytes are biconcave, anucleate cells specialized for oxygen transport. They do not divide and are renewed by stem cells in bone marrow.

• Function: Transport oxygen from lungs to tissues and carry carbon dioxide from tissues to lungs.

• Metabolism: Generate ATP anaerobically; do not consume transported oxygen.

Hemoglobin Structure and Function

Hemoglobin is the oxygen-carrying protein in erythrocytes, composed of four globin chains (2 alpha, 2 beta), each bound to a heme group containing iron.

• Oxygen Binding: Each iron atom binds reversibly to one oxygen molecule.

• Types: Oxyhemoglobin (oxygen-bound), Deoxyhemoglobin (no oxygen).

Erythropoiesis: Production of Erythrocytes

Erythropoiesis is the process of RBC production, regulated by erythropoietin (EPO) from the kidneys. It involves stem cell differentiation and maturation in bone marrow.

• Hemopoietic Stem Cells (HSCs): Multipotent cells in bone marrow that give rise to all formed elements.

• Colony-Forming Units (CFUs): Committed stem cells for specific blood cell lineages.

• Stages: Proerythroblast → Erythroblast → Reticulocyte → Erythrocyte.

• Regulation: Negative feedback; hypoxemia stimulates EPO release, increasing RBC production.

Example: High altitude or blood loss increases EPO secretion, boosting erythropoiesis.

Iron Metabolism in Erythropoiesis

• Iron Forms: Ferric (Fe3+) and Ferrous (Fe2+); only Fe2+ is usable for hemoglobin synthesis.

• Transport: Transferrin carries iron in blood; stored as ferritin in liver.

Erythrocyte Death and Anemia

RBCs have a lifespan of about 120 days. Anemia is a condition of reduced oxygen-carrying capacity, caused by decreased RBCs, hemoglobin, or defective production.

• Types: Hemorrhagic (blood loss), Hemolytic (RBC rupture), Iron-deficiency, Pernicious (B12 deficiency), Sickle cell.

• Symptoms: Fatigue, pale skin, shortness of breath, edema, reduced blood pressure.

Leukocytes and Immune Function

Types and Functions of Leukocytes

Leukocytes are nucleated cells involved in immune defense. They are less abundant than RBCs and classified as granulocytes or agranulocytes.

• Granulocytes:

  • Neutrophils: Phagocytize bacteria; most abundant.

  • Eosinophils: Combat parasites; phagocytize antigen-antibody complexes.

  • Basophils: Release histamine (vasodilation) and heparin (anticoagulant).

• Agranulocytes:

  • Lymphocytes: Adaptive immunity; produce antibodies, coordinate immune response.

  • Monocytes: Differentiate into macrophages; phagocytize pathogens and debris.

Leukopoiesis: Production of Leukocytes

Leukopoiesis is the formation of WBCs from hemopoietic stem cells in bone marrow.

• Myeloblasts: Form granulocytes.

• Monoblasts: Form monocytes.

• Lymphoblasts: Form lymphocytes.

Life Cycle: Granulocytes live 5 days; monocytes become macrophages and live years; lymphocytes provide long-term immunity.

Platelets and Hemostasis

Structure and Function of Platelets

Platelets are cytoplasmic fragments of megakaryocytes, essential for blood clotting (hemostasis).

• Functions: Secrete vasoconstrictors, form platelet plugs, release clotting factors, attract immune cells, destroy bacteria, stimulate vessel repair.

Hemostasis: Steps in Blood Clotting

Hemostasis is the process of stopping bleeding, involving several steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen, aggregate, and release chemicals to attract more platelets.

3. Coagulation: Clotting factors activate fibrin formation, creating a mesh that traps RBCs.

4. Clot Retraction: Clot contracts, serum is squeezed out.

5. Thrombolysis: Clot is dissolved by plasmin (activated by tPA).

Coagulation Pathways: Intrinsic (contact activation) and extrinsic (tissue factor) pathways converge to activate thrombin and fibrin formation.

Blood Types and Transfusion Compatibility

ABO and Rh Blood Groups

Blood types are determined by antigens (agglutinogens) on RBC surfaces and antibodies (agglutinins) in plasma. The ABO system is based on the presence of A and B antigens; the Rh system is based on the D antigen.

• Type A: A antigen, anti-B antibody

• Type B: B antigen, anti-A antibody

• Type AB: A and B antigens, no anti-A or anti-B antibodies (universal recipient)

• Type O: No antigens, both anti-A and anti-B antibodies (universal donor)

• Rh Positive: D antigen present

• Rh Negative: D antigen absent; anti-D antibodies form only after exposure

Blood Type Compatibility Table

Transfusion Reactions

If incompatible blood is transfused, antibodies bind to foreign antigens, causing agglutination (clumping) and destruction of RBCs. This can lead to serious complications.

Rh Factor and Pregnancy

Rh incompatibility can occur when an Rh-negative mother carries an Rh-positive fetus. Maternal antibodies may attack fetal RBCs in subsequent pregnancies, causing hemolytic disease of the newborn.

• Prevention: Rh immune globulin injections prevent antibody formation.

• Testing: Rh factor and antibody screens are performed during pregnancy.

Summary Table: Blood Components

Additional info: The included image depicts erythrocytes (red blood cells), highlighting their biconcave shape and abundance in blood. This structure increases surface area for gas exchange and flexibility for passage through capillaries.